Circuit arrangement comprising a power supply unit

ABSTRACT

The circuit arrangement comprises a power supply unit with a first power supply only for the analogue signal processing part and for the control part of a respective device. For the digital signal processing part, a second, separate switched mode power supply is provided, which is switched off, when only the analogue signal processing part is active, and which is in operation, when the digital signal processing part is on. For switching on and off of the second switched mode power supply, the power supply comprises a switching circuit ( 18 ), which is coupled in a special embodiment to a control loop ( 15 ) on the secondary side, for switching off the second power supply at the primary side via a galvanic isolating means ( 14 ). The switching circuit is provided with a supply voltage (12VS) from the first power supply and receives the on/off command from the control part, e. g. a microprocessor. The power supply unit is used advantageously within a D-VHS recorder, and the first power supply provides the power for the control part and the analogue circuit processing of the recorder. The second switched mode power supply is associated with the digital signal processing part. The second switched mode power supply is switched on for example, when a D-VHS tape is inserted into the recorder.

This application claims the benefit under 35 U.S.C. § 365 ofInternational Application PCT/EP01/14461 filed Dec. 10, 2001, whichclaims the benefit of European Application No. 00403651 .3 filed Dec.21, 2000,

FIELD OF THE INVENTION

The invention refers to a circuit arrangement comprising an analoguesignal processing part, a digital signal processing part and a powersupply unit, as used for example within devices of audio and videoequipment of consumer electronics. Devices of this kind incorporate moreand more digital electronics, but for essential parts still analoguecircuit processing is used, or is used in parallel to digitalelectronics. A new kind of video tape recorders for example, known asD-VHS recorders, are able to write and read data in analogue manneraccording to the VHS standard and to write and read digital dataaccording to the MPEG-2 standard in conformity to the D-VHS standard.

BACKGROUND OF THE INVENTION

A power supply unit comprising a first switched mode power supply onlyfor a standby mode operation and a second switched mode power supply fora normal mode of operation is disclosed in EP-A-0 803 966. Both switchedmode power supplies are synchronized with respect to the switchingfrequency and the second switched mode power supply is switched on viathe first switched mode power supply for normal mode operation.

SUMMARY OF THE INVENTION

The circuit arrangement according to the invention comprises a powersupply unit with a first power supply only for the analogue signalprocessing part and for the control part of the respective device. Forthe digital signal processing part, a second, separate switched modepower supply is provided, which is switched off, when the analoguesignal processing part is active. The second power supply is inoperation, when the digital signal processing part is active.

Within a D-VHS recorder for example, the first power supply provides thepower for the control part of the recorder, which comprises usually amicroprocessor for the control of the tape operations, for interpretingremote control commands, for the timer mode, and for the keyboard anddisplay drivers; and provides further the power for the analogue signalprocessing parts, which are responsible for writing and reading of theanalogue tape signals according to the VHS standard. For the digitalsignal processing part according to the D-VHS standard, a secondswitched mode power supply is responsible, which is in operation whenthe D-VHS mode is active, especially when a respective D-VHS recordingor reading is carried out. Finally, the digital signal processing partis also active when a digital signal is received through a digitalinterface, e.g. DV1394 IN.

For switching on and off of the second switched mode power supply, thepower supply comprises a switching circuit which is coupled in a specialembodiment to a control loop of this power supply on the secondary side,for switching off the second power supply at the primary side via agalvanic isolating means. The switching circuit is provided with asupply voltage from the first power supply and receives the on/offcommand from the control part, e. g. a microprocessor. The secondswitched mode power supply is switched on for example, when a D-VHS tapeis inserted into the recorder, and stays on, as long as the D-VHS taperemains in the recorder or as long the digital signal processing part isactive for signal improvement of signals played back from an analogueVHS tape.

The second switched mode power supply is especially a complete switchedmode power supply coupled to the mains AC voltage. In one embodiment ofthe invention it is arranged separately on a printed circuit board. Itcomprises advantageously also a mains filter for reducing switchingnoise. Therefore, when the digital signal processing part together withits switched mode power supply is off, there exists no additional noisefor the analogue signal processing part, which are very sensitive tonoise of a switched mode power supply.

Further, there is a considerable power consumption reduction as comparedwith a tape recorder with only one switched mode power supply, becauseboth switched mode power supplies work with optimum efficiency. Afurther important advantage of the two-power-supply concept is that thedigital signal processing part requires additional supply voltages,which can be easily provided by a second transformer. A power supplyunit with only one mains transformer would require many secondarywindings and additional output pins, which would demand for a specialdevelopment of a bulky mains transformer.

BRIEF DESCRIPTION OF THE DRAWING

The invention is explained in more detail below, by way of an example,with reference to a schematic circuit diagram, in which the only FIGUREshows a switched mode power supply for a digital signal processing part.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The switched mode power supply according to the FIGURE is arrangedwithin a circuit arrangement as a second power supply associated with adigital signal processing circuit. It comprises a transformer TR with aprimary winding W1, an auxiliary winding W3 on the primary side, and asecondary winding W2 on the secondary side, providing mains isolation asknown from current flyback converters. The input of the switched modepower supply is coupled to a 230 V AC mains voltage, which is filteredby a mains filter 1, rectified by a rectifier circuit 2 and smoothed bya charge capacitor C1 before applied to the primary winding W1 of thetransformer TR. In series to the primary winding W1 a switchingtransistor T1, for example a power MOSFET, and a sensing resistor R1 isarranged. In parallel to the primary winding W1 a clamp circuit 5 and inparallel to the switching transistor T1 a snubber network 3 is arrangedfor avoiding excessive voltage spikes, when the switching transistor T1switches off.

The switched mode power supply comprises a secondary regulation circuit15 coupled to the +5V output voltage, for providing output voltageregulation. The secondary regulation circuit 15 derives an error signalfrom the 5V output voltage, which is guided via a galvanic isolatingelement, an opto-coupler 14, to the primary side and coupled to thedriver circuit 8 of the switching transistor T1. As a further regulationinformation the voltage above the sensing resistor R1 is used.

Further control and protection circuits are arranged on the primary sideof the switched mode power supply, which information signals are allsummed up in a summing circuit 6. A summing circuit 6 of this kind isdisclosed for example in U.S. Pat. No. 5,867,373, which comprises acapacitor for summing up the currents. The information of the summingcircuit 6 is used by a switch off and control circuit 7, for driving theswitching transistor T1 via the driver circuit 8 by adjusting the dutycycle, so that the output voltages of the power supply, preferring the5V output, are regulated.

In detail, to one end of the auxiliary winding W3 an over-voltageprotection circuit 9 is coupled, which stops the switched mode powersupply directly via the driver circuit 8 in case of an over-voltage, anda T-ON control circuit 10. This T-ON control circuit defines the minimumduty cycle of the switching transistor T1 to ensure that the switchingtransistor T1 is always well saturated during the switching-on phase.

To the other end of the auxiliary winding W3 a control circuit 11, asoft start circuit 12 and a foldback point correction circuit 13 arecoupled. The control circuit 11 defines the timing of the switching-onof the switching transistor T1 with regard to the demagnetisation of thetransformer TR for switching through T1 in a minimum of the collectorvoltage. This switching principle is disclosed for example in U.S. Pat.No. 5,657,218.

The soft start circuit 12 provides a soft start-up of the power supplyafter switching on of the respective device. The foldback pointcorrection circuit 13 is a small circuit, which makes the maximum outputpower of the switched mode power supply independent of the amplitude ofthe mains voltage. A power good circuit 16, arranged at the secondaryside of the switched mode power supply, derives an information about themains voltage, which is used to stop the switched mode power supply whenthe mains voltage is too low.

The switched mode power supply according to the FIGURE is usedespecially as the power supply for the digital signal processing part ofa D-VHS video recorder. For the analogue signal processing part of therecorder, a further power supply, advantageously also a switched modepower supply which can be similar to the switched mode power supply ofthe digital part, is used. Both switched mode power supplies operateindependently, the second one only requiring an operating voltage, forexample 12 Volts, for the switching circuit 18. The ON/OFF command forswitching on and off of the second switched mode power supply isprovided by a micro-controller of the circuit arrangement, which ispowered by the first switched mode power supply and which is thereforealways on, when the recorder is on. The control of the circuitarrangement may be provided also by a system controller independent ofthe micro-controller. The micro-controller may provide the operatingvoltage for the switching circuit 18 only in a time interval, in whichthe ON-signal is applied to the second switched mode power supply.

The power good circuit 16 provides a control of the mains input voltageand provides a switch-off signal PG, when the mains input voltage is toolow. This control signal can be applied advantageously also to theswitching circuit 18, arranged at the secondary side, for switching theswitched mode power supply off, when the mains input voltage is too low.The power good circuit 16 generates the switch-off signal PG for exampleafter a mains interruption, with a delay of 20 msec. After a mainsinterruption, the ON/OFF signal applied to the switching circuit 18 mustbe switched off for about four seconds and then switched on again due tosafety reasons.

The circuit arrangement with a two-power-supply concept as describedabove is especially advantageous for a D-VHS video recorder. But furtherapplications, especially in the field of consumer electronics, as forexample set top boxes, CD- and DVD-players and -recorders, are alsowithin the scope of the present invention.

1. Circuit arrangement comprising an analogue signal processing part, adigital signal processing part, a control part including amicro-controller and a power supply unit, wherein said power supply unitcomprises: a first switched mode power supply for said analogue signalprocessing part and for said control part, and a second switched modepower supply with a transformer having a primary and a secondary sidefor providing power to said digital signal processing part, said secondswitched mode power supply comprising on the secondary side a switchingcircuit coupled to a control loop for output power regulation of saidsecond switched mode power supply, said switching circuit being providedwith a constant supply voltage from said first switched mode powersupply, said second switched mode power supply being switched on and offby said micro-controller via said switching circuit.
 2. Circuitarrangement according to claim 1, wherein said micro-controller switcheson and off said second switched mode power supply on the primary sidevia a coupling means.
 3. Circuit arrangement according to claim 1,wherein said first and second switched mode power supply comprise itsown start-up circuit and transformer for mains isolation.
 4. Circuitarrangement according to claim 3, wherein said second switched modepower supply is arranged separately on a printed circuit board andcomprises its own mains filter, which is coupled to a mains inputvoltage.
 5. Circuit arrangement to according to claim 1, wherein thecircuit arrangement is arranged within a D-VHS video recorder, and thefirst switched mode power supply is provided for supplying power to theanalogue reading and recording part, to the drive mechanism and to thecontrolling part of the recorder.